Electrically responsive devices are those devices that generate an output signal in response to an electrical signal and/or generate an electrical signal in response to an input signal. Typical types of input and output signals include electrical, optical, electromagnetic, vibrational, and thermal signals. One type of electrically responsive devices is a resonant device.
Resonant devices, such as surface acoustic wave devices, bulk acoustic wave devices, flexural plate/lamb wave devices, and quartz crystal microbalance devices are fashioned substantially from monolithic materials such as quartz, or, from layered materials that include uniform thin films of electroactive materials (e.g., piezoelectric materials such as Zinc Oxide, Lead Zirconate Titanate—PZT, Aluminum Nitride, Indium Nitride and Sol-Gel piezoceramic materials) in combination with other micromachinable materials (e.g., Silicon, Silicon Oxides, Silicon Nitride and Nickel Irons). These resonant devices, when preferentially coated with appropriate materials and/or packaged in an appropriate environment are used as, for example, electrical filters (e.g., passive), gas phase detectors, and liquid phase sensors.
As an electrical filter the resonant device is used to transmit resonant energy and filter signals outside of the pass band of the device. Electrical filters (e.g., surface acoustic wave (SAW) devices and film bulk acoustic resonator (FBAR) devices) are typically characterized by their insertion, transmission and reflectance properties. High transmission, isolated narrow band and stable properties of resonance operation are desirable features for filters used in modulating and demodulating wired and wireless signals. Defined and stable narrow pass band filter properties allow for packing more carrier frequencies in a given bandwidth.
Resonant devices also are sometimes used to determine the presence and amount of various measurands (e.g., chemical or biological) in gas phase and liquid phase samples. Resonant sensors may be used, for example, to determine the presence and magnitude of chemical vapors and biological matter in aerosol, the bulk properties of liquids (e.g., density, viscosity, and speed of sound) and the concentration of analytes in solutions.
In gas phase operation, a surface of a resonant device is typically coated with an absorbent material that interacts selectively and binds with specific gas phase products passed over the surface of the resonant device. The gas phase products that bind to the absorbent material increase the mass loading of the resonant device. The increase in mass loading changes properties of the device (e.g., the stiffness or the resonance of the device). Electrical signals produced by the resonant device reflect the change in resonance of the device.
In liquid operation, a surface of a resonant device is exposed to a liquid. The surface of the resonant device interacts with and is subsequently loaded by the physical interaction of the liquid with the resonant device. In some devices the loading of the resonant device occurs through the coherent oscillatory compression and motion of the liquid near the surface of the device. The resonant device produces an electrical signal that varies as the loading varies. A detectable resonance response results if the motion of the liquid is stable and oscillatory, and the oscillatory motion dissipates substantially less than the peak stored potential energy of the moving surface of the resonant device. Changes in the properties of the liquid are determined based on the electrical signal produced by the resonant device.
Generally, it is desirable for resonant devices to have isolated resonance modes, narrow pass bands, low loss, and stable and repeatable operating characteristics when used in a variety of operating environments (e.g., vacuum, gas phase, and liquid phase). Variations in the devices (e.g., due to manufacturing tolerances) tend to result in non-isolated resonance modes, wide resonance bands, high loss, and unstable and non-repeatable operating characteristics.